The present invention pertains to an apparatus and method for determining the distance between two points. More specifically, the present invention pertains to a three dimensional scanner which determines a distance between a known position and the surface of an object by measuring phase shift associated with collimated light transmitted from the known position, reflected, scattered, or dispersed off of the surface being measured, and received by an optical receiver associated with the collimated light transmitter, and more particularly to three-dimensional optical scanners.
The conversion of geometric data associated with a surface to digitized data is important for the generation of computer graphics. These computer graphics can be used in applications including, but not limited to, virtual reality, computer games, and multimedia presentations. The digitized data can be used for presentations regarding artwork, such as a museum being able to catalog or provide three dimensional models of famous sculptures without harming the original sculpture. Digitizing may also be used in other fields, such as manufacturing, where the determination of the shape of an object can be used to generate profiles for computer-aided machining, as well as for determining the dimensional accuracy of finished products.
The conversion of geometric data can be accomplished by the repetitive measuring of the distance between the surface and a known position using a device with an electrical/electronic output. Early computerized measuring machines (hereafter CMM""s) used mechanical probes, which were repetitively advanced into contact and retracted from a surface to determine the distance between the surface and a known location.
More recent devices have used a beam of light to determine the distance between the source and the surface. In U.S. Pat. No. 4,470,698 to Green, Jr., et al., a system is shown which uses a reflected beam of light for measuring distances. The Green reference is directed towards a system for optimally determining the orientation of counter-rotating optical wedges used to direct a beam of light for scanning purposes.
U.S. Pat. No. 4,698,498 to Mahoney et al. describes a three dimensional scanner which uses separate optical wedge pairs for scanning horizontal and vertical axes. By counter-rotating the optical wedges at a common rotational speed, the deflection of a light beam is constrained along a linear path. By using one pair of counter-rotating optical wedges to direct a generated light beam along a linear path, with the position of the path determined by the rotational position of the optical wedges. By reflecting the light off of an elevation mirror, the light transmitter can cause the beam to sweep a rectangular area. By reflecting returning light off of the elevation mirror and through a pair of optical wedges, the system can receive reflections of the light beam, and focus the beam onto a photoreceiver.
The prior art measurement of phase shift used in an application such as described in the Green patent relies on mixing the measured signal with a reference frequency signal, and determining the frequency difference. The phase difference is measured as a time difference between zero-crossing transitions of the measured periodic signal. Any noise in the system, however, can result in errors in the measured time difference. In order to obtain sufficient accuracy, the measurement of a particular distance must be either prolonged or multiply repeated to allow averaging of the time value to minimize the effect of any noise. The resulting impact on the system is an increase in the time required to obtain accurate distance measurements. When multiple measurements are required to derive a surface profile, and each measurement requires a longer dwell time, the overall scanning rate is reduced.
Also, in order to minimize the effect of noise on the signal measurement, band-pass filters are employed. The band-pass filters cause limited bandwidth, which results in time domain distortions of the measured phase shift. Accommodating these distortions also results in a reduction of the rate at which distance measurements can be accurately obtained.
The nature of available photoreceivers also inserts uncertainty into the system. The signal delay of the photoreceiver is dependent upon the intensity of the light received, and upon the distribution of the light intensity on the active surface of the receiver, which is defined by the angle between the scanning beam and the optical axis of the device.
In order to accommodate this dependence, it is necessary to know the phase, amplitude, and deflection angle of the light beam. As a result, measuring phase shift between electrical signals is not sufficient for accurate distance measurement. This problem is described in the article in Yakovlev W. W., Phase distortions in Germanium Avalanche Photo-Diodes, The Journal of Optical-mechanic industry (Rus), 1983, N12, p.13-14. This article provided the following equation for assessing the delay inherent in a photo-diode:       tg    ⁢          xe2x80x83        ⁢    ϕ    =            1      2        ·                  1        +                  3          ·                                    (                                                                    u                    0                                    -                                      i                    ·                    R                                                                    u                  1                                            )                        2                                                (                                                    u                0                            -                              i                ·                R                                                    u              1                                )                ·                  (                      1            -                                          (                                                                            u                      0                                        -                                          i                      ·                      R                                                                            u                    1                                                  )                            2                                )                      ·                            u          m1                +                  u          m3                            u        m1              ·                  u        m2                    u        1            
where:
"psgr" is the additional phase shift due to amplitude dependence
u0 is the offset voltage applied to the photo-diode
u1 is the threshold voltage of the photo-diode
um1, um2, and um3 are the amplitudes of the harmonic components of the received signal modulation
i is the average photo-current
R is the internal resistance of the photo-diode
The present invention overcomes these limitations by converting the output signal of the photoreceiver into a digitized signal, and comparing the digitized signal to a digitized reference differential signal corresponding to the generated light beam.
The optical scanner of the present invention may include a means for generating a first electronic modulation signal, a means for generating a second electronic modulation signal, and a means for generating a modulated beam of light, wherein the modulation of the light beam corresponds to the first electronic modulation signal.
A means for causing the modulated beam of light to be directed at a location on the surface to be measured is disposed between the means for generating a modulated beam of light and an object being measured. The means for causing the modulated beam of light to be directed at a location on the surface to be measured further provides a signal defining the orientation of the scanner to the object.
A means for receiving light from a surface being measured is disposed on the opposite side of the means for causing the beam of modulated light to be directed from the object being scanned. The means for receiving light includes a means for generating an output signal, which corresponds to the received light
A first mixer combines the first and second modulation signals to form a reference differential signal. A second mixer combines the output of the photoreceiver and the second modulation signals to form a mixed receiver signal. First and second analog-to-digital converters convert the reference differential signal and mixed receiver signal to form a digitized reference differential signal and a digitized mixed receiver signal.
A third analog-to-digital converter is provided to digitize DC and low frequency components of the raw receiver signal (photocurrent). The digitized raw receiver signal, digitized reference differential signal, and digitized mixed receiver signal are then communicated to a digital signal processor.
The digital signal processor determines a distance between the scanner and the object being scanned based on the phase difference between the digitized reference differential signal and the digitized mixed receiver signal. The digital signal processor then determines a correction factor to correct for delay inherent in photoreceivers by approximating the amplitude of the digitized mixed receiver signal and using this data in conjunction with the digitized raw receiver signal to determine the necessary correction, which is applied to the phase-derived distance to determine a more accurate distance. The distance value and position information identifying the location of the scanner derived distance are then communicated to an interface, so that the data can be communicated to a computer or other image processing element.
In a simple form, the invention is embodied in an apparatus for measuring the distance between two points having a photoreceiver which has an output signal. A converter compares the output signal to a digitized reference differential signal corresponding to a generated light beam. The comparison results in a resultant value which corresponds to the distance between the two points. The comparator further determines a correction factor for delays inherent in the photoreceiver. The correction factor applied to the resultant value provides a corrected resultant value.
Methodically, the invention is embodied in a process which involving converting an output signal from a photoreceiver into a digitized signal, and comparing the digitized signal to a digitized reference differential signal corresponding to a generated light beam. The comparison produces a resultant value, which corresponds to the distance between the two points. Also, a correction factor for delays inherent in the photoreceiver is determined based on the digitized output signal of the photoreceiver. This correction factor is applied to the resultant value to determine a corrected resultant value.